In the coming year we will critically examine whether the paternally derived X chromosome is functional before X inactivation occurs. For these studies we will use a thermolabile variant of alpha-galactosidase (Ags to the m power). We will determine patermal Ags expression by comparing heat inactivation kinetics of homozygous and heterozygous embryos. For these experiments we will examine heat inactivation kinetics insingle embryos, and pools of embryos. The latter will be a mixture of hemizygous males and heterozygous females. Thus, pooled assays will be a less sensitive test of differences than single embryos. We will also attempt to use H.Y antiserum as a means of comparing alpha-galactoisdase heat inactivation kinetics in pooled embryo samples in the different matin types. We will also extend our studies of the preferential expession of the X chromosome from the mother in extraembryonic tissues. Specifically, we will examine the mural trophoblast on the 8th and 9th day. For these studies, we wll utilize the electrophoretic variation for phosphoglycerate kinase (PGK-1) as a marker of X chromosome expression. These studies will establish the specific cell types that undergo preferential X chromosome inactivation.